Semiconductor lasers are typically fabricated on semiconductor wafers before they are cleaved into discrete laser devices. Semiconductor lasers may be edge-emitting lasers (EELs) or surface-emitting lasers (SELs). In EELs, the direction of light output is parallel to the wafer surface, while, in SELs, the direction of light output is perpendicular to the wafer surface. A SEL usually comprises an active region with one or more quantum wells for light generation. This active region, in turn, is sandwiched between upper and lower distributed Bragg reflector (DBR) mirrors. The DBR mirrors are constructed from a combination of semiconductor and dielectric layers with differing refractive indices. The upper DBR mirror typically has a lower reflectivity than the lower DBR mirror and is, therefore, the exit mirror through which the primary light output of the SEL is emitted.
SELs have various advantages over EELs for some applications including telecommunications and optical storage. A SEL, for example, typically has light output that is more circular and has a smaller divergence angle than the light output generated by an EEL. SELs may also be less costly to produce and more energy efficient. Finally, SELS are easier to test during manufacturing than EELs. This is due to the fact that EELs, unlike SELS, cannot be tested when they are still incorporated in a semiconductor wafer because of the parallel direction of light output.
The output power of a semiconductor laser is critically dependent on the operating temperature of that laser. As a result, automatic power control feedback methods are frequently implemented when using a semiconductor laser in order to maintain the laser's output power at a constant target value as the laser changes temperature. Automatic power control of a semiconductor laser is usually achieved by monitoring the output power of the laser using a monitor photodiode. The monitor photodiode generates an electrical output signal that is communicated to control circuitry external to the laser assembly. The control circuitry then determines the appropriate drive power that should be supplied to the laser in order to maintain the laser's output power at the constant target value.
However, automatic power control of SELs may be problematic. Conventionally, in order to implement automatic power control in a SEL, several optical devices, such as mirrors, beam splitters, lenses and the like are positioned in the optical path of the primary light output. These optical devices act to steer a portion of the primary light output to a nearby monitor photodiode which, in turn, sends an electrical output signal to the control circuitry. Nevertheless, these optical devices are frequently difficult to position in a reproducible manner from laser assembly to laser assembly. As a result, monitor photodiodes in laser assemblies of the same type will frequently see differing amounts of light for a given laser output power. Moreover, the optical devices used for monitoring the primary light output of SELs are relatively expensive, attenuate the output of the SEL being monitored, and frequently produce unsatisfactory power control.
There is, as a result, a need for an easily fabricated, inexpensive SEL assembly that allows automatic power control without using the primary light output of the SEL to monitor output power.